1. Field of the Invention
The present invention relates to highly sensitive and specific immunoreactant assay reagents. More particularly, the present invention relates to assay reagents and systems wherein a sensitized solid reagent is provided and used in combination with a particular buffer reagent that reduces non-specific antigen-antibody reactions. The assay reagents of the present invention are designed to avoid the need for sample pretreatment and be compatible with commercial autoanalyzers typically used in the field of clinical chemistry.
2. Discussion of the Background
Digoxin is a popular cardiac glycoside currently prescribed for the control of congestive heart failure and for certain cardiac rhythm abnormalities. The increased cardiac output resulting from the ionotropic action of digoxin ameliorates the disturbances characteristic of heart failure such as venous congestion, edema, dyspnea, orthopnea and cardiac asthma. Digoxin also reduces ventricular rate and thus improves hemodynamics. Palpitation, precardial distress or weakness are relieved and concomitant congestive failure ameliorated. Digoxin also slows the heart and induces regular sinus rhythm. Regardless whether digoxin is used to control/inhibit heart failure, atrial fibrillation or flutter, the continued administration of digoxin after the onset of clinical event is typically recommended.
The therapeutic index for digoxin is very low, there being only a very narrow difference between therapeutic and toxic dosages. Digoxin levels in patients are often difficult to predict because of variation in the absorption of oral doses and the variation and non-renal excretion. Accordingly, the monitoring of serum digoxin levels is a valuable and necessary tool in decreasing patient toxicity risk and in detecting underdigitalization. This is particularly true since the incidence of toxicity increases from 5 to 71% for serum digoxin levels of 1.1 and 4.4 mg/mL, respectively.
Current digoxin immunoassay techniques include radioimmunoassay (RIA) systems, enzyme linked immunosorbent assays (ELISA) and EMIT assays. In radioimmunoassay systems, digoxin is generally labeled with radioactive iodine and the amount of labeled digoxin bound to an antibody is measured with a gamma-ray counting instrument. Such RIA systems present several drawbacks, such as the use of radioactive elements, sample instability, significant reagent preparation, expensive measuring instruments, etc.
ELISA and EMIT assays also require the labeling of digoxin, albeit with an enzyme, and the subsequent monitoring of an enzyme-substrate reaction. These assay systems, like the RIA assay, require significant reagent preparation, etc. Moreover, current assays systems typically include a serum pretreatment step in order to destroy interfering proteins which contribute to non-specific (i.e., serum interferant-digoxin antibody) reactions and lead to false positive results. For example, when performing an EMIT digoxin assay, the serum or plasma is mixed with sodium hydroxide to destroy interfering proteins.